don't install already installed python3

Author: kszucs

dev/tasks/gandiva-jars/travis.osx.yml

@@ -53,7 +53,7 @@ script:
   - dev/tasks/gandiva-jars/build-cpp-osx.sh || travis_terminate 1
   - dev/tasks/gandiva-jars/build-java.sh || travis_terminate 1
   # deploy using crossbow
-  - brew install libgit2 python3
+  - brew install libgit2
   - pip3 install click github3.py jinja2 jira pygit2 ruamel.yaml setuptools_scm toolz
   - >
     python3 dev/tasks/crossbow.py